Aim: Beta diversity describes the variation in species composition between sites and can be used to infer why different species occupy different parts of the globe. It can be viewed in a number of ways. First, it can be partitioned into two distinct patterns: turnover and nestedness. Second, it can be investigated from either a species identity or a functional&#8208;trait point of view. We aim to document for the first time how these two aspects of beta diversity vary in response to a large environmental gradient.

Location: Maloti&#8208;Drakensberg Mountains, southern Africa.

Methods: We sampled ant assemblages along an extensive elevational gradient (900&ndash;3000&nbsp;m a.s.l.) twice yearly for 7&nbsp;years, and collected functional&#8208;trait information related to the species&rsquo; dietary and habitat&#8208;structure preferences. We used recently developed methods to partition species and functional beta diversity into their turnover and nestedness components. A series of models were used to test whether the observed beta diversity patterns differed from random expectations.

Results: Species beta diversity was driven by turnover, but functional beta diversity was composed of both turnover and nestedness patterns at different parts of the gradient. Null models revealed that deterministic processes were likely to be responsible for the species patterns but that the functional changes were indistinguishable from stochasticity.

Main conclusions: Different ant species are found with increasing elevation, but they tend to represent an increasingly nested subset of the available functional strategies. This finding is unique and narrows down the list of possible factors that control ant existence across elevation. We conclude that diet and habitat preferences have little role in structuring ant assemblages in montane environments and that some other factor must be driving the non&#8208;random patterns of species turnover. This finding also highlights the importance of distinguishing between different kinds of beta diversity.

jbi12537-fig-0004: Plot of the first two principal coordinate axes of functional trait spaces occupied by the ant fauna of the Sani Pass, southern Africa. Regions where turnover (darker red) and nestedness (darker blue) dominate are highlighted. The density of turnover and nestedness occurrence throughout the space was calculated separately and then combined to produce a single gradient describing the dominance of either pattern. Turnover was defined as areas present in a higher elevations that were not present in the lowest elevation. Nestedness was defined as areas present in higher elevations that were also present in the lowest elevation.

Mentions:
Across our entire 7‐year dataset, there is consistency in where changes in the functional structure of assemblages take place. Functional turnover is largely seen through the introduction of species with traits indicating predatory specialization and life in open habitats (negative values on axes 1 and 2, Fig. 4). Functional nestedness results in extreme trait combinations being lost from the functional space. This leaves the assemblages at the highest elevations with species that possess generalized traits centred on the origin (0, 0) of the functional trait space. These species tend to be dietary generalists with no strong preference for open or closed environments (Figs 1, 4). These patterns could be reflecting deterministic community‐assembly processes. Environmental conditions may selectively filter which species are able to successfully establish and survive at each elevation. Such filtering would act on the functional trait values of the species. For this case, it would seem that species with extreme trait values are not able to exist at the highest elevations. This idea is consistent with the clustering and shrinking of phylogenetic diversity at higher elevations (Machac et al., 2011; Hoiss et al., 2012).

jbi12537-fig-0004: Plot of the first two principal coordinate axes of functional trait spaces occupied by the ant fauna of the Sani Pass, southern Africa. Regions where turnover (darker red) and nestedness (darker blue) dominate are highlighted. The density of turnover and nestedness occurrence throughout the space was calculated separately and then combined to produce a single gradient describing the dominance of either pattern. Turnover was defined as areas present in a higher elevations that were not present in the lowest elevation. Nestedness was defined as areas present in higher elevations that were also present in the lowest elevation.

Mentions:
Across our entire 7‐year dataset, there is consistency in where changes in the functional structure of assemblages take place. Functional turnover is largely seen through the introduction of species with traits indicating predatory specialization and life in open habitats (negative values on axes 1 and 2, Fig. 4). Functional nestedness results in extreme trait combinations being lost from the functional space. This leaves the assemblages at the highest elevations with species that possess generalized traits centred on the origin (0, 0) of the functional trait space. These species tend to be dietary generalists with no strong preference for open or closed environments (Figs 1, 4). These patterns could be reflecting deterministic community‐assembly processes. Environmental conditions may selectively filter which species are able to successfully establish and survive at each elevation. Such filtering would act on the functional trait values of the species. For this case, it would seem that species with extreme trait values are not able to exist at the highest elevations. This idea is consistent with the clustering and shrinking of phylogenetic diversity at higher elevations (Machac et al., 2011; Hoiss et al., 2012).

Aim: Beta diversity describes the variation in species composition between sites and can be used to infer why different species occupy different parts of the globe. It can be viewed in a number of ways. First, it can be partitioned into two distinct patterns: turnover and nestedness. Second, it can be investigated from either a species identity or a functional&#8208;trait point of view. We aim to document for the first time how these two aspects of beta diversity vary in response to a large environmental gradient.

Location: Maloti&#8208;Drakensberg Mountains, southern Africa.

Methods: We sampled ant assemblages along an extensive elevational gradient (900&ndash;3000&nbsp;m a.s.l.) twice yearly for 7&nbsp;years, and collected functional&#8208;trait information related to the species&rsquo; dietary and habitat&#8208;structure preferences. We used recently developed methods to partition species and functional beta diversity into their turnover and nestedness components. A series of models were used to test whether the observed beta diversity patterns differed from random expectations.

Results: Species beta diversity was driven by turnover, but functional beta diversity was composed of both turnover and nestedness patterns at different parts of the gradient. Null models revealed that deterministic processes were likely to be responsible for the species patterns but that the functional changes were indistinguishable from stochasticity.

Main conclusions: Different ant species are found with increasing elevation, but they tend to represent an increasingly nested subset of the available functional strategies. This finding is unique and narrows down the list of possible factors that control ant existence across elevation. We conclude that diet and habitat preferences have little role in structuring ant assemblages in montane environments and that some other factor must be driving the non&#8208;random patterns of species turnover. This finding also highlights the importance of distinguishing between different kinds of beta diversity.